Project Summary/Abstract Autoimmune disease often occurs because CD4+ T cells do not become tolerant to major histocompatibility complex II-bound peptides from the host?s proteins (self p:MHCII). The textbook view is that immature thymocytes with high affinity TCRs for self p:MHCII complexes displayed by thymic antigen-presenting cells undergo apoptosis or become suppressive regulatory T (Treg) cells. This mechanism applies to self p:MHCII generated from proteins that are expressed by thymic antigen-presenting cells naturally, or ectopically under the control of the AutoImmune Regulator transcription factor. Some self p:MHCII complexes are thought to be displayed in the secondary lymphoid organs but not the thymus and are tolerated because mature T cells with high affinity TCRs become anergic. Although this is an elegant theory, it has not been validated because a natural population of polyclonal T cells that becomes anergic in the extra-thymic tissues of normal individuals as a consequence self p:MHCII recognition has not been identified. Here, we will test the hypothesis that the newly discovered population of Foxp3? FR4high CD73high T cells is the natural anergic population. We will determine whether these cells become anergic via persistent stimulation by self p:MHCII complexes that are displayed in secondary lymphoid organs but not the thymus and can regain some degree of responsiveness under hyper-inflammatory conditions and participate in organ-specific autoimmunity. This project could provide the best evidence to date that T cell anergy is a physiological tolerance mechanism and identify the types of self epitopes that it protects the host from. It could validate or rewrite the textbooks and be of applied value by improving understanding of how tolerance fails during autoimmune disease.